The destruction of breeding habitats is one of the most basic ways to combat mosquitoes.
PHOTO: FOTOLIA/HENRIK LARSSON
Using nature to solve natural problems is a sound idea, this article shares a number of chemical-free mosquito control ideas.
A mosquito develops through four stages—egg, larva, waterborne pupa, and emerging adult—and it's helpful to know where the life cycles of various types of mosquitoes take place in order to understand how to deal with our particular insect problem. Species can be grouped more or less into three categories, according to breeding-site preferences.
The Permanent Pool Group: Most species in this class inhabit bodies of fresh water—particularly quiet pools with abundant surface vegetation. Such spots include not only permanent ponds but also semi-permanent sites such as roadside ditches, pits, canals, clogged streams, and irrigated lands. Breeding is continuous, and several generations are produced each year. The adults' flight range extends one to two miles tom the breeding site. Species in this group can carry encephalitis, dog heartworm, and in some cases malaria.
The Floodwater Group: These mosquitoes usually appear early in the season, with most species producing several broods per year. They breed in receding pools, on damp soil in grassy depressions, or along grassy shorelines that are intermittently flooded. The eggs are laid, the wet area dries out, and large numbers of larvae hatch with the next flooding. The adults' flight range extends five to twenty miles from the breeding site, and species in this group have been known to carry dog heartworm.
The Artificial Container/Tree Hole Group: These insects prefer to lay their eggs in small, water-filled places—birdbaths, such litter as old cans and jars, and other man—made containers, as well as holes in trees. Species in this group produce several broods per year. The eggs are laid on the inner surface of the container or tree hole at or above the waterline, and they hatch when inundated after a dry period. In general, the habits of these mosquitoes are similar to those of the floodwater group.
MOSQUITOES AND PHYSICAL CONTROL
The destruction of mosquito breeding habitats is one of the most basic ways to combat them using this chemical-free mosquito control method. Several courses of action, such as the following, can be taken.
 Making a pond's shallow shore steeper or deeper is effective, since it's unlikely that mosquitoes will breed in water over six inches deep. Dredging to this depth or attaining a slope ratio of 3:1 has been found to deter the establishment of vegetation and breeding conditions.
 Remove weed growth in shore areas to destroy the larvae's protection and food source.
 Fill in depressions and potholes with soil or other compactible material to eliminate these breeding areas.
 Drain standing water. This can be accomplished through open ditching, the use of subsurface drains, or pumping.
 Empty all miscellaneous standing-water sources. Any receptacles that hold water-old bottles, tin cans, water troughs, barrels, children's wading pools, and undrained boats should be emptied, removed, covered, or turned upside down. Change the water in pet dishes daily and birdbaths twice weekly, washing them as you do so. Either discard old tires or store them indoors. Screen any rain barrels and openings to water tanks, cisterns, and wells. Repair leaky plumbing and outside faucets, and check drain outlets from air conditioners. Clean leaf-clogged roof gutters and drain flat roofs. Finally, fill holes in trees with sand or mortar, or drain them when necessary.
MOSQUITOES AND BIOLOGICAL CONTROL
Biological control doesn't work as rapidly as chemicals, but it can be longer-lasting and environmentally safer. For this reason, some people encourage mosquito-eating toads to take up residence in their yards, while others build special birdhouses for such insect-gobblers as purple martins. On the southern coast of Maine, dragonflies—which dine on mosquito larvae and adults both—are utilized instead of poisonous sprays. There are, however, other mosquito predators and parasites that may not be as familiar.
For ponds large and deep enough to support fish, several mosquito-controlling species are available:
The mosquito fish (Gambusia affinis) is considered to have the best potential. It's only about two-and-a-half inches long, but it's been credited with reducing mosquito propagation in ponds by 80% to 95%. Use ten to twelve fish for a pool with a surface area of about ten square yards. They reproduce quickly, but automatically slow their birth rate when their density threatens to outrun the supply of mosquito larvae and other food. They like shallow areas with little current and will tolerate a fair amount of pollution. They don't, however, survive northern winters, so a yearly stocking program would have to be implemented in cold-weather areas.
Fathead minnows (Pimephales promelas) are primarily bottom and midwater feeders and are good for mud-bottomed, shallow waters such as flooded fields, ponds, and ditches. They need water temperatures of 64 degrees Fahrenheit to spawn and can produce several broods per season.
Golden shiners (Notemigonus crysoleucas) like clear, well-vegetated ponds, though they will tolerate some pollution and can live in warmwater swamps. They need 60 degrees Fahrenheit to 80 degrees Fahrenheit water to produce young, which feed primarily on algae and microcrustaceans. These fish are especially good for small farm ponds.
Goldfish (Carassius auratus) prefer clear, quiet pools with submerged vegetation, though they, too, can survive a moderate amount of pollution and a high content of organic matter. They don't spawn until they are about two years old but are prolific after that. Naturally, they are perfect for ornamental ponds, though they are not effective on certain mosquitoes of the permanent pool group.
Guppies (Poecilia reticulata) are similar to goldfish but are not as tolerant of environmental extremes and cannot survive temperatures lower than 52 degrees Fahrenheit. They are prolific, producing a brood of 60 to 80 offspring every 28 days.
To obtain any of these fish, it's best to check with your state Department of Conservation or local fish farms. Some are also available as bait species or can be obtained from mosquito-control contractors around the country.
BETTING ON BACTERIA
The use of bacteria in biological control has proved quite successful. Bacillus thuringiensis israelensis (Bti) has been widely effective as a larvicide. It was approved by the EPA in 1981 as effective on some 30 species of mosquitoes. The bacterium does not seem to harm other aquatic life or mammals, but results in a 90% to 100% kill on most types of mosquito larvae. When sprayed over water, it's eaten by larvae. Knockdown activity begins within a few hours, and total kill takes place within 24 hours. Bti will remain active in the water for up to three days; after this, it will die, too. Thus, it doesn't endanger the ecology of the area by persisting and reproducing. However, it's effective only on larvae.
Bti comes under several trade names and is applied either in liquid, granular, or briquette form. The amount applied to a given area varies with the density of vegetation, the number of larvae, and other environmental factors detailed on the product label. Generally, it's most effective when the larvae have just hatched.
In addition to local dealers and contractors, you can contact the following manufacturers for product information:
Biochem Products, Montchanin, DE. Brand name: Bactimos (liquid, granular, and briquette forms).
Abbott Laboratories, Dept. 95-M, North Chicago, IL. Brand name: Vectobak (liquid and granular forms).
Bti in briquette, liquid, and granular forms; mosquito fish; and large-scale contracting services are available from Clarke Outdoor Spraying, Roselle, IL.
For a free copy of the 55-page publication Mosquitoes of Public Health Importance and Their Control (Public Health Service, 1982), write to Centers for Disease Control, Center for Infectious Diseases, Parasite Diseases Division, Atlanta, GA 30033.